The Pacific Decadal Oscillation, or PDO, affects sea surface temperatures and wind flow in the North Pacific. This graphic from NASA shows sea surface temperature departures from average, as well as wind anomalies (arrows), for different phases of the PDO. Credit: NASA JPL.

The Bering Sea, west of Alaska, gives us yet another example of “Global Weirding” – not Global Warming, with some of the coldest temperatures on record in the past four years. Chinook salmon yields have “jumped” recently in Oregon and Washington, with the third-highest recorded in 2010. Pacific Northwest salmon thrive in cooler waters, suffering as temperatures rise.

Until about four years ago, the Bering Sea was warming up like most of the world’s oceans. “It was extremely warm there not that long ago, and now we’ve got a few years that are really cold in the context of a climate still really warm globally,” says Nathan Mantua, a climate scientist at the University of Washington.

“The reason you can have these shifts regionally is because atmospheric circulation changes, and sometimes you get dramatic regional changes that go against global trends.”

Research has shown that the Pacific Decadal Oscillation (PDO)* — a long-term atmospheric circulation pattern — is largely responsible for the Bering Sea’s recent temperature flip-flop, though other factors also play a role.

Here’s what PDO looks like – a veritable “yo-yo” of temperature swings over the decades:

Time series of shifts in the phase of the Pacific Decadal Oscillation (PDO), from 1925 to 2010. Red bars indicate positive (warm) years; blue bars indicate negative (cool) years. Credit: NOAA PMEL.

Wind seems to play an important role here:

“It’s almost as simple as which way the wind blows,” says Mantua. Every summer, the Bering is ice-free, but as colder weather arrives, ice forms at the border between the sub-Arctic and the Arctic. How far south it extends into the Bering Sea each year depends in large part on how far the winter winds push it. Scientists call this “sea ice advection.”

“More wind out of the north brings especially cold air that drives the [winter sea] ice to lower latitudes,” says Mantua. “If the winds are mostly out of south, they’re bringing mild air from lower latitudes to higher latitude parts of the Arctic.” A study led by Jinlun Zhang of the University of Washington recently confirmed that the PDO, combined with winter sea ice advection, might explain much of the variation in the Bering’s water temperatures.

Winter sea ice extent, in turn, affects the entire ecosystem. More winter sea ice means a large “cold pool” exists during spring and summer — a frigid footprint left behind by the melted sea ice. In warm winters with little sea ice, as in 2000 through 2005, the cold pool can be virtually nonexistent. This affects everything from the tiniest zooplankton to marine mammals, fish, and seabirds.

“The next question is, what is it about climate that causes changes in wind?” asks Mantua. “The surface wind variability is strongly influenced by the pressure difference between the high pressure area in Eastern Russia and the Aleutian Low. The PDO is well correlated with the Aleutian Low, but not the Russian High. So the PDO is a significant part of the story, but not the only part of the story.”

There’s an important message here:

…there’s a lot of regional variation that may be completely independent of global warming.

A multidecadal pattern of climate variability centered across the North Pacific Ocean. During the positive (warm) phase of the PDO, sea-surface temperatures tend to be above average along the west coast of North America and in the eastern tropical Pacific; while across the central North Pacific they are cooler than average. The opposite patterns occur during the negative (cool) phase. Each phase typically persists for 20 to 30 years, with a warm phase predominating since the late 1970s. The PDO may be related to ENSO, but differs mainly because the timescale for the PDO is much longer (several decades) and because the PDO more clearly involves the extratropical Pacific and the Aleutian Low pressure system.

The North Pacific Index (NPI) is a measure of the average sea-level pressure over the North Pacific. It is strongly related to the PDO and has associated teleconnections across North America, which are labeled the Pacific/North American Pattern.